The present invention relates to an umbilical for use in the offshore production of hydrocarbons, and in particular to a power umbilical for use in deep water applications.
An umbilical consists of a group of one or more types of elongated or longitudinal active umbilical elements, such as electrical cables, optical fibre cables, steel tubes and/or hoses, cabled together for flexibility, over-sheathed and, when applicable, armoured for mechanical strength. Umbilicals are typically used for transmitting power, signals and fluids (for example for fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.
The umbilical cross-section is generally circular, the elongated elements being wound together either in a helical or in a S/Z pattern. In order to fill the interstitial voids between the various umbilical elements and obtain the desired configuration, filler components may be included within the voids.
ISO 13628-5 “Specification for Subsea Umbilicals” provides standards for the design and manufacture of such umbilicals.
Subsea umbilicals are installed at increasing water depths, commonly deeper than 2000 m. Such umbilicals have to be able to withstand severe loading conditions during their installation and their service life.
The main load bearing components in charge of withstanding the axial loads due to the weight (tension) and to the movements (bending stresses) of the umbilical are steels tubes (see for example U.S. Pat. No. 6,472,614, WO93/17176, GB2316990), steel rods (U.S. Pat. No. 6,472,614), composite rods (WO2005/124095, US2007/0251694), steel ropes (GB2326177, WO2005/124095), or tensile armour layers (see FIG. 1 of U.S. Pat. No. 6,472,614).
The other elements such as the electrical and optical cables, the thermoplastic hoses, the polymeric external sheath and the polymeric filler components, do not contribute significantly to the tensile strength of the umbilical.
The load bearing components of most umbilicals are made of steel, which adds strength but also weight to the structure. As the water depth increases, the suspended weight also increases (for example in a riser configuration) until a limit is reached at which the umbilical is not able to support its own suspended weight. This limit depends on the structure and on the dynamic conditions at the (water) surface or ‘topside’. This limit is around 3000 m for steel reinforced dynamic power umbilicals (i.e. umbilical risers comprising large and heavy electrical power cables with copper conductors).
However, it is desired to create power umbilicals for ultra deep water (such as depth (D)>3000 m). Such umbilicals comprise very heavy copper conductor cables and must be strongly reinforced to be able to withstand their beyond-normal suspended weight and the dynamic installation and operating loads. An easy solution would be to reinforce such umbilicals with further steel load bearing strength members, such as the rods, wires, tubes or ropes described above. However, due to the specific gravity of steel, this solution now also adds a significant weight to the umbilical. In static conditions, the water depth limit of this design is around D=3200 m, where the maximum tensile stress in the copper conductors of the power cables (being weak point of the structure) reaches its yield point (at the topside area close to the surface). However, in any dynamic conditions, this depth limit is naturally lower because of the fatigue phenomenon. Depending on the waves, on the floating production unit movements, and on the kind of bend stiffener which is used, the limit of this design in dynamic conditions is between 2700 m and 3000 m.
Furthermore, such steel reinforced umbilicals are very very heavy and require evermore powerful and expensive installation vessels.
A suggested solution to this problem consists in using composite material strength members shown by WO2005/124095 and US2007/0251694. However, such umbilicals are difficult to manufacture and so are very expensive.
GB2326177A discloses a deep water umbilical comprising a large central steel cable 4 surrounded by helically wound fillers and peripheral steel tubes 2″. In the lower section, this assembly is replaced by a large steel tube 5. However, the cable-tube transition is very complex and difficult to manufacture. The helical peripheral tubes 2″ must also be connected to the large central tube 5 through a manifold which is also used for transmitting the tensile load to the large central cable 4.